According to the Universal Serial Bus (USB) specification, host USB devices can power other (e.g., peripheral) devices. Host USB devices, such as personal computers and notebook computers, can provide up to but not more than 500 milli-amperes (mA) of electrical current to a peripheral device such as a portable storage device (e.g., a portable hard disk drive).
Contemporary portable storage devices (e.g., 2.5-inch portable hard disk drives) can consume up to 1000 mA of current during read or write operations, not including the amount of current consumed by the USB-to-SATA (Serial Advanced Technology Attachment) bridge integrated circuit. Clearly, the amount of current needed by a portable storage device can exceed the limit included in the USB specification.
To overcome this problem, some types of portable storage devices are equipped with a USB Y-cable that allows the drive to be plugged into two USB ports at the same time. A portable storage device can thus draw up to 500 mA of current from one USB port and up to another 500 mA of current from another USB port, thereby obtaining the amount of current needed for read and write operations. However, this type of solution may be problematic because the host device may not have two available USB ports. That is, the host device's USB ports may be needed for other purposes, and it may not be possible to devote two ports to a single device such as a portable storage device.
Other types of portable storage devices are instead equipped with a power adapter that allows them to be plugged into an external power supply (e.g., an electrical outlet). However, this type of solution is cumbersome and inconvenient, especially for a storage device that is supposed to be portable. In other words, a power adapter can be relatively bulky, and its use requires that the portable storage device be placed with reach of an electrical outlet (e.g., a wall outlet).